According to canon, is there anything preventing one from concealing one's Horcrux inside a building/place and then simply protecting it with the Fidelius charm, with one being the one and only secret-keeper?
It seems huge flaw to me that V allowed his Horcruxes to be found (and eventually destroyed) that easily. Especially the ring and locket. They were both in some kind of rooms. Using the Fidelius charm would make them simply unobtainable, thus making big V REALLY immortal.
According to cannon, no, there is nothing to prevent one from concealing one's horcruxes inside a building place then simply protecting it with the Fidelius Charm. In fact this has happened.
After Grimmauld Place was made the Order headquarters and Dumbledore put his Fidelius charm over it, Slytherins locket which was one of the horcruxes was still in there, making it a horcrux inside the boundaries of a Fidelius Charm, showing that it's not impossible
As for why Voldemort didn't do it, please see here.
